[imp] [ALERT] Fatal error: Invalid argument

[Paul.Reilly at tcd.ie]

I'm getting a

[ALERT] Fatal error: Invalid argument

when ever logging in to mailboxes with more messages than can be
displayed on one page. I can't seem to find a fix for this problem.
I have put two screenshots of what happens, up here:

http://www.tcd.ie/People/Paul.Reilly/

Symptoms:
1) only happens on first page of multi-page mailbox
2) happens on inbox and other mailboxes
3) calculates page numbers correctly, but fails to display messages

ie: it gives the error, and says there are 3 screenfulls of messages, but
fails to display the first batch of messages. If I append '&page=1' to the
query string it works, but this shouldn't be neccessary. Also in normal
behaviour it doesn't have the page number in the query string.

I've tested this in IMP 3.2.1 and 3.2.2 and can reprodce the problem.
Has anyone seen this before?
Any tips on where to look for a fix?
I've checked through mailbox.php but everything there looks fine.

Paul